This is mostly a theory post with some need of verification and testing. It assumes OAx weapons still interact with footwork as they once did and Footwork has no limitation beyond 80% haste cap regarding maximum achievable attack speed. If either of those have been shown to no longer be true, then the rest of this post is irrelevant. There's a TL;DR at the end btw!
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As far as I am aware, H2H's max speed is limited based on base H2H delay of 480 + the delay of a weapon. So, Verethragna can at most reduce its delay to 106 and Spharai can reduce it to 113. At Lv99, starting with a delay of 280 (Martial Arts VII), it only requires approximately 68% haste to achieve the minimum delay. This is generally reachable using max gear haste (256/1024) + max magic haste (448/1024) for a total of 68.75% (704/1024) which most groups / people strive toward always having. MNK does not generally have access to JA haste unless getting Haste Samba via sub or other party member, but it would not improve the speed any further. This limitation causes H2H to lose its natural speed advantage against other weapons since they can make use of the extra haste.
There was a time where Footwork was popular to use with OAx weapons due to OAx's odd interaction with determining whether or not multi-kicks occur and Kick WS using kick boot damage. However, due to increasing h2h weapon damage and new/better WS, it was no longer viable.
Since then, gear has improved allowing for more multi-attacks than before. This post explains theoretically how to determine the chance of multi-attacking with Footwork active. I assume triple/quadruple attacks would count in the calculation between double attacks and OAT although they would only produce 1 extra kick. MNK/WAR can cap gear haste while having 28%+ double attack (10% from sub included), with some form of triple/quadruple attacks thrown in depending on what is worn. Assuming OAT has 40% double attack chance, it should be possible for double kicks to occur nearly 86% of the time if the equation still applies.
Assuming Footwork can make use of ~78.6% (805/1024) haste, it would attack faster than standard h2h if it only uses 480 delay to determine its max speed. Combined with OAT weapon if it still interacts the same way as it used to, it could WS much more frequently while having some base damage to help with WS damage output.
TL;DR - Could Footwork w/ OAT99 and current gear + ~78% (magic/gear/samba) haste produce a viable amount of damage compared to standard h2h at capped haste (ignoring how probable it is to achieve the setup and assuming anyone would want to do math regarding it)?
Edit: How does mythic OA2-3 aftermath interact with Footwork? Perhaps that could be compared as well.